This invention relates to optically modulated overwritable recording devices by which the new information can be written directly over the old information upon a magneto-optical recording medium.
A conventional optically modulated overwritable magneto-optical recording medium is disclosed, for example, in Japanese patent application No. 1-154918. In the case of such a overwritable magneto-optical medium, the old information is erased simultaneously as the new information is recorded on the medium. FIG. 36 shows the stable recording temperature ranges or temperature patterns for recording information upon the recording medium. As shown in FIG. 36, the medium exhibits two temperature threshold levels T.sub.C1 and T.sub.C2. When recording information in the form of 1's and 0's, the laser power is modulated at two levels. The "1" is recorded at a high level above the threshold level T.sub.C2 (e.g., at the peak levels of the temperature patterns 1, 3 and 4), while "0" is recorded at a low level between the two threshold levels T.sub.C1 and T.sub.C2 (e.g., at the peak levels of the temperature patterns 2 and 5). When either "1" or "0" is recorded, the old information previously recorded upon the recording medium is erased simultaneously. When the laser power is below the threshold level T.sub.C1, no recording or erasure is effected (temperature pattern 6).
In the case of the non-overwritable medium, on the other hand, the old information is erased by reversing the externally applied magnetic field before the new information is recorded upon the medium. Thus, the laser power for recording "1" and "0" are substantially at the same levels. As a result, the non-overwritable medium exhibits wide margins for the permissible laser power levels.
In the case of the optically modulated overwritable medium illustrated in FIG. 36, on the other hand, three distinct levels must be distinguished: the high power level for recording "1" (the first recording power level); the intermediate level for recording "0" (the second recording power level or the so-called erasure power level); and the low power level for reproducing the recorded information (the reproduction power level). Thus, the margins or allowances of the laser power levels for recording "1" and "0" are much narrower.
FIG. 37a shows the allowable recording power range of a non-overwritable magneto-optical recording medium. Due to the variation of the sensitivity to the recording laser power level, the allowable recording laser power range differs from one medium to another. The allowable range also varies with the change in ambient temperature. The margin or the allowable range of the recording laser power level for each medium, however, is sufficiently wide. Thus, the common allowable range for all the media put to use over the service temperature range exhibits a substantial width as represented by the white arrow (at the legend `the allowable range for write "1"`) at the bottom of FIG. 37a. The laser power level for recording information can thus be set without difficulty.
FIG. 37b shows the allowable recording power ranges of an overwritable magneto-optical recording medium. Since the power level for recording "0" must be distinct from the power level fop recording "1" the allowable ranges of the recording laser power levels for each medium are narrowed. Thus, the common allowable ranges of the recording laser power levels for writing "1" and "0" for all the media put to use over the service temperature range are extremely narrow, as represented by two short white arrows (at the legends `allowable range for write "0"` and `allowable range for write "1"`) at the bottom of FIG. 37b.
It is thus difficult to determine the appropriate power levels for recording "1" and "0" upon an overwritable magneto-optical recording medium. An expensive spectrum analyzer is necessary for measuring and determining the optimal recording power levels for recording information upon the overwritable medium. Namely, the threshold level T.sub.C2 of the overwritable medium is determined by the level of the recording characteristic, and in particular is obtained as the range of the high laser power level in which the ratio C/N of the carrier level to the noise level N exceeds a reference level. On the other hand, the threshold level T.sub.C1 is determined by the level of the erasure characteristic, and is obtained, for example, as the range of the laser power level satisfying the following condition. Assume that a signal f.sub.1 is recorded upon the medium, and then a new signal f.sub.2 is written over the old signal f.sub.1 recorded on the medium. Then, the remnant amount of the signal f.sub.1 should be below a predetermined reference level. The predetermined reference level of the remnant amount after erasure is, for example, from -40 dB to -25 dB, and a spectrum analyzer is necessary for measuring the amount of the feeble remnant old signal f.sub.1 embedded in the newly recorded signal f.sub.2. The spectrum analyzer is thus needed for determining the optimal recording power levels of the overwritable medium.
A method of controlling the recording and reproduction upon this kind of optically modulated overwritable medium is disclosed, for example, in Japanese Laid-Open Patent (Kokai) No. 3-116566. According to this Japanese patent, the optimal laser power levels for the recording, the reproduction, and the erasure are determined by evaluating 1) the error rate of recording data 2) the DC recording level, and 3) the test signal recording level. The evaluations ate effected with the high and the low laser power levels as the parameters. Thus, according to this Japanese patent, the overwriting characteristic is comprehended in terms of the recording and the erasure processes, and the high laser power level and the low laser power level are varied linearly to determine the optimal laser power levels. However, it is difficult to determine the optimal recording power levels with high precision and with a high degree of reliability when the sensitivity of the recording medium or the ambient temperature varies. For example, the evaluation of the error rate is time-consuming. For the purpose of obtaining the bit error rate of 10.sup.-6, as many as 10.sup.7 data bits must be monitored. Thus, this method is not suitable for the quasi-real time processing which is required when booting the device or exchanging the recording medium. Further, with respect to the recording of the test signal, the evaluation of the erasure characteristic is effected by monitoring the erasure level. The precise measurement of the erasure level, however, is difficult.
In summary, the conventional optically modulated overwritable recording device has the following disadvantage. Since the appropriate recording laser power levels are extremely narrow and the measurements thereof may be adversely affected by the adjustment errors and the variation in the ambient temperature, the determination of the optimal laser power levels is difficult and requires an expensive spectrum analyzer, and even using a spectrum analyzer, it is sometimes impossible.